The present invention relates to printed wiring boards and specifically to protection against damage of electrical components and traces on the printed wiring board due to high temperature, fire, plasma and debris.
It is common to place the electronic components of transient surge suppression (TVSS) devices on printed wiring boards. Transient Voltage Surge Suppressors are generally constructed using voltage-clamping devices known as Metal Oxide Varistors (MOVs). The principal of operation of these devices is well documented in manufacturers"" literature. Generally, these devices are used in shunt with a pair of conductors and act as voltage clamps, working against the impedance of the circuit, to limit the voltage at the point of connection.
It is well known that MOV devices can be damaged by continuous overvoltage (voltage that causes excessive power dissipation). In addition, MOV devices have a finite life expectancy when operated in an environment where surge voltages and currents are present. Both conditions display a similar failure mode in that a thermal avalanche within the MOV material occurs. For either failure mode (continuous overvoltage or end-of-service), the MOV clamping voltage decreases as the material heats, clamping voltage is lost, and the MOV becomes a low impedance path. Since the MOV is generally placed in shunt with a power source, the low impedance path causes significant current flow from the power source that can cause severe thermal overheating and possible bursting of the MOV device. Should a catastrophic failure occur, causing the MOV to burst, plasma will most likely be generated. In a confined space, this plasma can cause sustained arcing that will release tremendous amounts of thermal energy until a protective device operates to remove current from the circuit. Since the arc itself has some amount of impedance to the flow of current, the operation of the overcurrent protective devices is slowed. At the same time, the overtemperature devices must warm up to operating temperature in order to react. The release of thermal energy can vaporize organic (printed wiring boards) and metal (component leads and PWB traces) present in the vicinity of the arc. In the case of metal that is connected to the same electrical circuit as the arc, a transfer of the arc from the plasma to the energized metal can occur. Vaporization of organic materials by the arc generally liberates basic atomic constituents of the material that can recombine and liberate additional thermal energy thereby creating additional damage.
To minimize the damage to surrounding circuitry or apparatus during a MOV failure, various methods have traditionally been employed. Overtemperature devices that act to interrupt power supplied to the MOV have been used but generally have a slow response time. Overtemperature devices can only protect the MOV in situations where the fault current during failure is sufficiently low that immediate bursting of the MOV material does not occur. The use of overcurrent protection devices in series with the MOV can help prevent rupture of the MOV material during failure by limiting the overall energy flow into the MOV. However, the overcurrent protection device must not inadvertently operate during normal operation of the MOV. This usually means that the coordination of the overcurrent protection device with the bursting energy of the MOV is very difficult and for larger MOV arrays, not possible. To create coordination between the overcurrent protection device and MOV, a containment system is generally employed around the MOV. The function of the containment system is to manage the thermal and bursting effect of the MOV (emission of flame, fire, plasma, debris and products of combustion) until the overcurrent protection device can react. A containment method taught in U.S. Pat. No. 5,488,534, incorporated herein by reference as typical prior art, requires the MOV""s to be placed on separate printed wiring boards which are located in compartments separated from other electronic components by barriers. The compartments containing MOV""s are filled with compacted silica (sand). The compacted sand is then covered with an epoxy, which is also used to pot the electronic component compartments. Although compacted sand will absorb some of the plasma expelled from a bursting MOV it does not provide an adequate barrier to arcing at the MOV leads and vaporizing of the adjacent PWB. It would be advantageous to provide an easily installed means for protecting the MOV leads, printed wiring board and other electronic components from effects of arcing and plasma expelled by exploding MOV""s. A more compact containment device is disclosed in U.S. patent application Ser. No. 09/067,118 filed on Apr. 27, 1998, and is hereby incorporated by reference. This containment device includes features other than compacted sand that surround the MOV""s to prevent damaging missiles from being expelled from the TVSS device during a catastrophic failure. These features permit the MOV""s and other TVSS electronic components to be placed on a common printed wiring board within a common enclosure. The enclosure is filled with potting material to further restrain the effects of a catastrophic failure and to prohibit circuit tampering. It is desirable to provide protection from catastrophic failure of a device such as a MOV without having to use potting material and possibly eliminate the need for compacted sand around the MOV. It is also desirable that the containment method be usable with relatively small enclosures and permit the use of one printed wiring board for all electronic components of the TVSS device.
The present invention provides protection to a printed wiring board (PWB) and the electrical components mounted on the PWB from high temperature, fire, plasma and debris resulting from a catastrophic failure of an at-risk electronic component, such as a MOV, also located on the PWB or in close proximity to the PWB. This protection is provided by a layer of inorganic refractory coating applied to a surface of the PWB adjacent the at-risk electrical component. The refractory material is preferably applied to the PWB surface as a pourable liquid. The refractory material must have sufficient viscosity such as to easily flow in and around electronic components located on the PWB and be self-leveling before curing, thus providing a uniform thickness. The refractory material should also have a small shrinkage factor when cured such that no gaps are created between the PWB and the enclosure and around the electrical terminals of the at-risk components. It is also desirable that the coating be capable of curing independent of the applied thickness.